Lithium manganese composite oxides (hereinafter referred to as Li—Mn composite oxides), which are very safe and are produced from abundant natural resources, have been of interest for use as a cathode electroactive material for lithium ion secondary cells. However, Li—Mn composite oxides exhibit poor discharge capacity per amount of an electroactive material as compared with lithium cobalt composite oxides (hereinafter referred to Li—Co composite oxides). In addition, secondary particles of Li—Mn composite oxide are lightweight and absorb a large amount of oil, because the particles contain many pores. Thus, the amount of electroactive material which can be fed into a dimensionally limited cell must be restricted, thereby disadvantageously lowering the electrochemical capacity of a unit cell.
In recent years, U.S. Pat. No. 5,807,646 (Japanese Patent Application Laid-Open (kokai) No. 9-86933) has proposed measures to counter the aforementioned problem. Specifically, a mixture of a manganese compound and a lithium compound is shaped at a pressure of 500 kg/cm2 or higher, heated, and crushed, to thereby produce an Li—Mn composite oxide having a tapping density (i.e., apparent density of powder in a container which is moved, e.g., vibrated under certain conditions) of 1.7 g/ml or higher. However, the disclosed tapping density is at most 1.9 g/ml, which is unsatisfactory.
The above official gazette also discloses the average particle size of secondary particles which are formed by aggregating primary particles of an Li—Mn composite oxide. However, even when the packing density of secondary particles is enhanced through the interaction between primary particles, secondary particles are disintegrated during the electrode material (paste) preparation step. Thus, controlling the average particle size of the secondary particles is not a fundamental counter-measure.
Some methods for producing a spinel-type Li—Mn composite oxide have already been disclosed. Japanese Patent Application Laid-Open (kokai) No. 9-86933 discloses such a method comprising burning a mixture of a manganese compound and a lithium compound at a high temperature, e.g., 250° C. to 850° C. Japanese Patent Application Laid-Open (kokai) No. 4-237970 discloses such a method comprising mixing a manganese compound, a lithium compound, and an oxide of boron which can be substituted by manganese and burning the resultant mixture at a high temperature, to thereby produce an Li—Mn—B oxide in which Mn atoms are partially substituted with B, and the Li—Mn—B oxide serves as a cathode electroactive material.
When the aforementioned materials are burned at high temperature in air or in an oxygen gas flow, the secondary particles obtained through crushing have a high average porosity (15% or more) and a low tapping density (1.9 g/ml or less). Thus, thus-obtained cathode electroactive materials cannot be charged into an electrode in a large amount, and thereby, a high discharge capacity cannot be attained.
Japanese Patent Application Laid-Open (kokai) No. 4-14752 discloses a cathode electroactive material employing a manganese oxide which is produced by mixing spinel-type lithium manganese oxide and titanium oxide and sintering the resultant mixture. However, disadvantageously, titanium oxide only reacts with lithium and manganese at 950° C. to 1000° C. or higher to form a melt, and a tapping density of 1.60 g/ml can be only attained by adding titanium oxide in an amount as large as 10 mass %.